As is known, a radial tyre comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread and a belt positioned circumferentially between the carcass reinforcement and the tread. This carcass reinforcement is made up in the known way of at least one ply (or “layer”) of rubber which is reinforced with reinforcing elements (“reinforcers”) such as cords or monofilaments, generally of the metallic type in the case of tires for industrial vehicles which carry heavy loads.
To reinforce the above carcass reinforcements, use is generally made of what are known as “layered” steel cords made up of a central layer or core and one or more concentric layers of wires positioned around this core. The three-layered cords most often used are essentially cords of M+N+P construction formed of a core of M wire(s), M varying from 1 to 4, surrounded by an intermediate layer of N wires, N typically varying from 3 to 12, itself surrounded by an outer layer of P wires, P typically varying from 8 to 20, it being possible for the entire assembly to be wrapped with an external wrapper wound in a helix around the outer layer.
As is well known, these layered cords are subjected to high stresses when the tires are running along, notably to repeated bendings or variations in curvature which cause rubbing on the wires, notably as a result of contact between adjacent layers, and therefore to wear, as well as fatigue; they therefore have to have high resistance to what is known as “fretting fatigue”.
It is also particularly important for them to be impregnated as far as possible with the rubber, for this material to penetrate into all the spaces between the wires that make up the cords. Indeed, if this penetration is insufficient, empty channels or capillaries are then formed along and within the cords, and corrosive agents, such as water or even the oxygen in the air, liable to penetrate the tires, for example as a result of cuts in their treads, travel along these empty channels into the carcass of the tyre. The presence of this moisture plays an important role in causing corrosion and accelerating the above degradation processes (the so-called “corrosion fatigue” phenomena), as compared with use in a dry atmosphere.
All these fatigue phenomena that are generally grouped under the generic term “fretting corrosion fatigue” cause progressive degeneration of the mechanical properties of the cords and may, under the severest running conditions, affect the life of these cords.
To alleviate the above disadvantages, application WO 2005/071157 has proposed three-layered cords of 1+M+N construction, particularly of 1+6+12 construction, one of the essential features of which is that a sheath consisting of a rubber composition covers at least the intermediate layer made up of the M wires, it being possible for the core (or individual wire) of the cord itself either to be covered or not to be covered with rubber. Thanks to this special design, not only is excellent rubber penetrability obtained, limiting problems of corrosion, but the fretting fatigue endurance properties are also notably improved over the cords of the prior art. The longevity of the tires and that of their carcass reinforcements are thus very appreciably improved.
However, the described methods for the manufacture of these cords, and the resulting cords themselves, are not free of disadvantages.
First of all, these three-layer cords are obtained in several steps which have the disadvantage of being discontinuous, firstly involving creating an intermediate 1+M (particularly 1+6) cord, then sheathing this intermediate cord using an extrusion head, and finally a final operation of cabling the remaining N (particularly 12) wires around the core thus sheathed, in order to form the outer layer. In order to avoid the problem of the very high tack of uncured rubber of the rubber sheath before the outer layer is cabled around the core, use must also be made of a plastic interlayer film during the intermediate spooling and unspooling operations. All these successive handling operations are punitive from the industrial standpoint and go counter to achieving high manufacturing rates.
Further, if there is a desire to ensure a high level of penetration of the rubber into the cord in order to obtain the lowest possible air permeability of the cord along its axis, it has been found that it is necessary using these methods of the prior art to use relatively high quantities of rubber during the sheathing operation. Such quantities lead to more or less pronounced unwanted overspill of uncured rubber at the periphery of the as-manufactured finished cord.
Now, as has already been mentioned hereinabove, because of the very high tack that rubber in the uncured (uncrosslinked) state has, such unwanted overspill in turn gives rise to appreciable disadvantages during later handling of the cord, particularly during the calendering operations which will follow for incorporating the cord into a strip of rubber, likewise in the uncured state, prior to the final operations of manufacturing the tyre tread and final curing.
All of the above disadvantages of course slow down the industrial production rates and have an adverse effect on the final cost of the cords and of the tires they reinforce.